This invention relates to a communication system in which individual stations are connected to a control center by a transmission medium common to all of the stations to allow the transmission of messages in the form of digital signals between the control center and the individual stations. The length of a message transmitted to or from a specific station is determined by the information content of that message and is independent of the length of other messages to or from the same station or other stations. In particular, the invention relates to stations for use in such a system, and especially to means provided in each station to allow modular expansion units to be connected directly to the station or to another modular expansion unit that is connected, directly or indirectly, to the station. The modular expansion units make it possible for the information-handling capability of each station to be tailored to requirements specific to that station without burdening other stations with the need to provide such additional capability.
One well-known type of signal distribution system is a community antenna television (CATV) system in which television signals are applied to a transmission medium, usually a coaxial cable, by equipment at a central station that is commonly known as a "head end." These television signals are transmitted along the cable and can be picked up by television receivers connected to terminals at spaced points on the cable. In a simple form of CATV system, the television signals are transmitted in only one direction, called the downstream direction, away from the head end, and no circuits and provided by which the receiver can be controlled from the head end.
More sophisticated systems provide for additional signals to be transmitted downstream to control the individual receivers so as to determine which television signals can be received by each of the receivers. This requires control circuits at each subscriber's receiver, but the requirements are the same for each installation. It is not necessary to provide means for expanding the signal-handling capability of any of the receivers relative to others.
It has also been proposed to use the wideband facilities of a CATV network to transmit large quantities of data between two points on the network. The equipment at each station may be quite different from that at any other station, and the advantage of having relatively simple, basic equipment with provision for modular expansion to equipment capable of handling additional information or processing it differently has not been explored.
Still other digital communication systems operate on a synchronous, or bit-oriented, basis in which a clock signal is maintained continuously. This has the advantage of permitting a simple synchronizing signal to be used to indicate the start of each message, but it has the distinct disadvantage of requiring no idle time between messages. Synchronous systems can be more efficient than byte-oriented asynchronous systems in the transmission of long messages but are not as flexible in the spacing and arrangement of messages.